In connections of medium voltage electrical cables, gaps or interstices must not be present in the insulating material surrounding the core connection or electrical stresses can degrade and destroy the connection. This requirement must be met under varying operating conditions over long time periods of operation. Particular difficulties are encountered with oil-filled, paper-insulated medium voltage cables because the oil level thereof may undergo changes during manufacture of the cable connection or in use, and because the paper insulation which normally is wrapped, is delicate and can be easily damaged.
Cast spacer bodies are often used as the insulating material surrounding a core connection. They are manufactured at the site of installation in a mold placed around the core connection. That technique is, however, expensive in time and labor and requires a relatively large amount of casting compound. Yet this will not completely eliminate the danger that gaps and interstices will form during operation; and this is particularly true at the boundary between the casting compound and the oil in oil filled cables.
West German Offenlegungschrift Nos. 29 09 060 and 30 21 845 and European Pat. No. 0017953 disclose medium voltage electrical cable connections for oil-filled paper-insulated medium voltage cables in which the end of one cable is arranged in a hermetically sealed metal casing which is filled with oil or casting compound. That cable is connected via a hermetically sealed insulated plug connection with the second cable which is normally a plastic-insulated cable. That technique is also very expensive.
In another cable connection for an oil-filled cable, heat-shrinkable sleeves are used for making oil-tight connections. However, in that method the heating that is necessary for the shrinking causes thermal expansion of the oil thereby creating an oil loss in the end portion of the oil-filled cable. Undesirable cavities may form where the oil loss occurs. Also, the necessity to employ a heat source which can be easily handled, normally in the form of an open flame, is often inconvenient or inadmissible. Finally, the manufacture of such cable connections requires considerable experience and skill of the assembling personnel, and the quality of the result of the work is highly dependent upon the personal abilities of the installer.